The environmental costs of fossil fuels and the political instabilities of oil-producing regions have intensified efforts to develop alternative energy sources that are environmentally clean and more reliable. Wind-driven power generation systems are of particular interest. Wind power may be converted to electrical power using a rotor assembly, either horizontally or vertically oriented. The rotor blades convert the energy of the moving air into a rotational motion of a drive shaft. An electrical generator coupled to the drive shaft then converts the rotational motion into electrical power. Typically, a fixed-ratio gear box converts the low rotation speed of the rotor assembly to a higher rotation speed for the electrical generator.
A conventional wind-driven power generation system is typically a monopole tower with a single rotor rotating about a hub located at or near the top of the tower. The tower produces power only when the wind blows, only within a certain range of wind velocities, and at a maximum power output level for an even smaller range of wind velocities. As a result, wind power generation has traditionally been expensive to produce and not reliably available. In response, conventional wind turbine manufacturers' designs have evolved towards very large rotor assemblies and very tall towers in order to gain economies of scale and to reach higher velocity and steadier winds at higher altitudes.
However, a larger rotor assembly rotates more slowly than a smaller rotor assembly and requires a higher gear ratio to provide an optimal rotational speed range for the electrical generator. A larger rotor assembly also has a greater mass, requiring stronger winds to cause rotation. Furthermore, a larger rotor assembly applies greater torque stress to a gear box, requiring that the gear box be larger in size and made of more exotic and expensive materials. Finally, even with exotic materials and sturdier supports, a larger rotor assembly is still limited to a lower maximum wind speed at which the rotor assembly can operate without causing damage to the mechanical components of the wind tower.
An augmented wind power generation system uses a funneling apparatus, for example a fully or partially shrouded rotor, to increase the velocity of the ambient wind across a smaller rotor assembly. Such funneling apparatuses may be vertically stacked into a tower with one or more rotor assemblies located in each apparatus. Such wind amplification devices are described in U.S. Pat. No. 4,156,579 (Weisbrich), U.S. Pat. No. 4,288,199 (Weisbrich), U.S. Pat. No. 4,332,518 (Weisbrich), U.S. Pat. No. 4,540,333 (Weisbrich), and U.S. Pat. No. 5,520,505 (Weisbrich). All five Weisbrich patents are hereby incorporated by reference as if fully set forth herein.
The wind speed amplification effect of the funnel permits power generation to occur at lower ambient wind speeds. Specifically, because the electrical power generated from wind is a cubic function of the wind's velocity, a smaller rotor assembly can generate similar amounts of power to a larger rotor with an equal amount of ambient wind. In other words, the rotor assemblies of an augmented wind power generation system are typically smaller than those in a traditional wind tower, and therefore have a smaller mass and higher rotational speeds. As a result, some augmented turbines rely on low rpm generators and require no gear box while others require a gear box with lower gear ratios that are made of more conventional, less expensive materials.
Although providing benefits over a larger rotor assembly, both configurations of augmented wind power generation systems described above continue to be constrained by inefficiencies in gear box and or generator performance. As a result, energy is lost due to the use of less than ideal generators or to a limitation of wind speed environments in which the turbine and gear box assembly can operate without causing damage to the components of the system. Therefore, there is a need in the art for an improved apparatus and method for converting energy from wind into electrical power.